Reverse osmosis (RO) membranes are semipermeable barriers which, when pressurized on one side with a solution such as aqueous solution of sodium chloride (NaCl), allow preferential passage of the solvent (water) to separate solvent from the solute (NaCl). The important operating characteristics of an RO membrane are salt rejection, that is, the degree to which the dissolved salt is separated from the solvent, and flux, that is, the amount of solvent that permeates the membrane in a given time at a given pressure.
Thin-film composite RO polyamide membranes are typically made by interfacial polymerization. This is carried out by immersing a microporous polymeric substrate in an aqueous diamine followed by immersion in a solution of a polyacyl halide in an organic solvent. The organic solvent typically is chosen based on its ability to dissolve the polyacyl halide to a useful concentration while not interfering with the polyamidation reaction and not damaging the microporous substrate membrane. The solvent of choice has been 1,1,2-trichlorotrifluoroethane (FREON.RTM.TF), also referred to herein, generically, as CFC-113. CFC-113, however, is expensive and, being a chlorofluorocarbon (CFC), has been cited as potentially causing undesirable environmental effects.
The use of additives in organic solvents is described in U.S. Pat. No. 4,761,234. In that patent, the additive is N,N-dimethylformamide and the solvent is CFC-113. The additive functions as a catalyst for the polyamidation reaction.
Hexane as well as other aliphatic hydrocarbons, such as heptane and naphtha, are free of the potential environmental concerns associated with CFC solvents. These hydrocarbons also are known to be useful solvents for interfacial polyamidation. See, for example, U.S. Pat. Nos. 4,005,012; 4,259,183; 4,360,434; 4,606,943; 4,737,325; and 4,828,708. However, the membranes produced by interfacial polyamidation with hexane as the reaction solvent typically have lower flux than membranes produced with CFC-113. The use of hexane as a reaction solvent, therefore, has not gained wide commercial acceptance.
If one could substitute aliphatic hydrocarbons such as hexane for CFC solvents in the manufacture of thin-film composite RO membranes and still manufacture membranes with flux levels comparable to those attained with CFC solvents, the process would be less expensive and environmentally advantageous. The process of this invention meets that objective.